In the course of examining cells isolated from rats with clinical signs of EAE, the principal investigator noted the presence of a cell surface marker (OX40) on CNS-resident T-cells. This antigen, a member of the TNF-R family, was for the most part exclusively expressed on MBP-specific T- cells and primarily on those that had homed to the CNS, leading the principal investigator to propose in this new R01 that OX40 is key surface molecule on EAE-effector cells. His plan is to assess the biological significance of the OX40 expression, using anti-OX40 reagents to define recently activated T-cells in the inflammatory site, to costimulate these cells via their OX40 receptor, to identify the OX40 ligand within the target CNS tissue, to prepare an immunotoxin and use it in a demyelinating relapsing EAE model, to characterize human OX-40 expression in MS, and finally, to characterize the in vivo significance of OX40 on CD4 T-cells by creating a mouse knockout for this receptor. The identification of EAE-selective markers on T-cells is a very worthwhile goal, given the state of the art in immunosuppressive therapies for MS, and the particular approach presented in this application is rational, timely, and within the scope of the skills of this investigator.